Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

"You can think of your dog not only as your best friend, but also a social support tool for being active," study researcher John Sirard, an assistant professor at the University of Virginia in Charlottesville, said in a statement.

Sirard and his colleagues surveyed 618 pairs of Minneapolis adolescents and their parents about the number of dogs in their home, and how much time they spent doing physical activities. For one week, 318 of those teens also wore accelerometers, which are wrist watch-like devices that collect data regarding how and when the body is moving.

Teens from dog-owning families recorded greater amounts of movement on the accelerometers, even after researchers took into account demographic variables, such as gender, race and socioeconomic status.

The finding was unexpected because the researchers anticipated the dog-walking responsibilities would be taken by the parents. "We hypothesized it would have an effect on adults, but we didn't see that. We saw it in the kids," Sirard said.

Despite the link between dog ownership and teenagers' physical activity, researchers said they could not be certain that getting a dog would encourage people to be more active. It could be that more active people choose to have dogs, because the pet already fits their lifestyle, Sirard said.